Reproductive isolation is a biological process necessary for the formation of distinct species. It involves mechanisms that prevent gene flow between two populations, ensuring they remain genetically separate. Postzygotic barriers are mechanisms that take effect after a hybrid zygote, or fertilized egg, has successfully formed between two different species. These barriers reduce the viability or fertility of the resulting hybrid offspring, blocking the successful continuation of the hybrid lineage and maintaining the separation between parent species.
When Hybrid Development Fails
Reduced hybrid viability is the first type of postzygotic barrier, meaning the hybrid offspring has a lower chance of survival compared to the purebred offspring of either parent species. Although fertilization occurs, the combination of genes from the two different species is often incompatible. This genetic mismatch disrupts the coordinated processes required for normal embryonic development, causing the hybrid to be frail or fail to develop past early stages.
The resulting hybrid embryo may spontaneously abort, or the offspring may be born with developmental abnormalities that make survival to reproductive age unlikely. For example, when certain frog species mate, their hybrid tadpoles often fail to complete metamorphosis or die shortly after hatching. Similarly, some hybrid salamanders suffer from developmental issues that prevent them from reaching adulthood. This failure ensures that even if two different species successfully mate, their genetic material is not integrated into the population.
Sterile Hybrid Offspring
Reduced hybrid fertility is another form of postzygotic isolation. The hybrid organism survives to adulthood but is unable to produce functional reproductive cells (gametes). This barrier prevents the hybrid from passing its genes on to the next generation, stopping gene flow between the parent species. The most common cause for this infertility is the inability of the parent chromosomes to align properly during meiosis, the cell division process that creates sperm and eggs.
The classic example is the mule, the hybrid offspring of a female horse and a male donkey. A horse has 64 chromosomes, while a donkey has 62, so the mule inherits 63 chromosomes. Since chromosomes must pair up precisely during meiosis, the odd number and structural differences make alignment impossible, leading to non-functional sperm or eggs. This mismatch ensures the mule is almost always sterile and cannot breed with other mules or back-cross with either parent species.
Failing Subsequent Generations
Hybrid breakdown is the third mechanism, a delayed form of reproductive isolation apparent only after the first hybrid generation. The initial first-generation hybrids (F1 generation) are often viable and fully fertile. These F1 hybrids can successfully mate with each other or with one of the parent species, but problems emerge in the second (F2) or later generations.
The subsequent generations inherit new combinations of parental genes that are incompatible, leading to reduced survival or fertility. This occurs because the initial F1 generation received a balanced mix of chromosomes. When these chromosomes are reshuffled in the F2 generation, the resulting genetic combinations can be severely maladaptive. For instance, in crosses between different subspecies of rice, the F1 generation is fertile, but the F2 generation often shows significant weakness, stunted growth, or complete sterility. A similar effect is observed in crosses of cotton, where specific recessive genes cause a lethal breakdown phenotype in later generations.